Fiber optic bundles having randomly distributed light source fibers and reflected light fibers have been utilized for angular displacement measurements by irradiating a moveable, non-deformable surface with light radiated from the ends of the light source fibers and measuring light reflected from the surface that irradiates the ends of the reflected light fibers. Such devices have been used for angular measurements of rotating surfaces having fixed reflective and non-reflective portions. As the non-reflective portion of the surface covers a larger part of the angle subtended by the fiber optic bundle end, the intensity of the reflected light decreases in proportion to the displacement of the surface with respect to the fiber optic bundle end. However, use of this type of fiber optic bundle has been limited to displacement measurements of solid surfaces having variable reflective characteristics. Accurate and repeatable calibration of surface displacement as a function of reflected light intensity has required reflecting surface to be non-deformable.
In contast to fiber optic-angular displacement transducers described above, conventional fiber optic pressure transducers have been constructed from a plurality of discrete optical fiber bundles have a predetermined geometrical orientation with respect to each other. Various geometrical orientations have been proposed in order to obtain maximum transducer response. In one configuration, a central fiber bundle is surrounded by six additional fiber bundles. The central bundle transmits light from a light source to a pressure-sensitive diaphragm having a reflective surface. The six bundles surrounding the central bundle transmit light reflected from the diaphragm to an intensity measurement device such as a photocell. The amount of reflected light depends on the distortion of the diaphragm due to a pressure differential across the diaphragm surfaces. There are three disadvantages to a conventional system utilizing well organized, discrete fiber optic bundles. These are (1) fabrication requires careful assembly with attendant high cost, (2) calibration is very sensitive to the relative position of the fiber bundles with respect to the reflecting diaphragm and the orientation of the light source with respect to the central bundle; and (3) a well organized array of discrete fiber optic bundles carries the characteristics of the light source for a significant distance. It has been found that 90 centimeters of fiber are required to average out the effects caused by an incandescent light bulb filament, resulting in a bulky transducer and high cost.
The optical pressure transducer of the present invention eliminates the above described problems by utilizing, in conjunction with a deformable diaphragm having a reflective surface, an optical fiber bundle having randomly distributed light source fibers and reflected light fibers. A large number of optical fibers randomly distributed are utilized to define light source and refleted light fibers combined into a single fiber optics bundle at one end and branched into two bundles at the opposite end. The single bundle end is positioned respective a pressure-sensitive, light-reflecting diaphragm. The branched end of the combined bundle defines a light source bundle and a reflected light bundle. The end of the light source bundle is irradiated by a light source and conducts light to the diaphragm. The reflected light bundle conducts light reflected by the diaphragm. A photocell or the like measures light radiated from the reflected light bundle end. By randomly selecting the light source bundle optical fibers and the reflected light bundle optical fibers from the combined bundle, there is obtained a random distribution of optical fiber types in an end cross section of the combined bundle. It is preferred to use very thin optical fibers, on the order of 0.001 inch in diameter so that the combined bundle will have at least 100 optical fibers and in many cases 500 or more.
There are many advantages of an optical pressure transducer constructed according to the present invention. A lower manufacturing cost is possible because the light source optical fibers and reflected light optical fibers are selected randomly from the combined bundle of optical fibers, thereby eliminating the need to position them according to a predetermined configuration. A further result of random selection is that calibration of the reflected light intensity as a function of diaphragm deformation is not sensitive to the relative locations of individual fibers within the combined bundle. Accordingly, one calibration curve can be used for different transducers of the same type while still maintaining a high degree of accuracy. An additional advantage is that, within practical bounds, there is virtually no limitation on the shortness of the fiber optic bundle since the random distribution of the individual fiber optics destroys the image characteristics of the light source.